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## Summary of the Pull Request
So this PR adds a profile setting called "confirmCloseAllTabs", that allows one to enable or disable the "Do you want close all tabs?" dialog that appears when you close a window with multiple open tabs. It current defaults to "true". Also adds a checkbox to that dialog that also sets "confirmCloseAllTabs"
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## References
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## PR Checklist
* [x] Closes#3883
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [ ] Tests added/passed
* [ ] Requires documentation to be updated
* [ ] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx
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## Detailed Description of the Pull Request / Additional comments
I added a checkbox to the close dialog to set this setting, but I'm not sure how to best go about actually changing the setting from code; am open to suggestions, as to how it should be done, or if I should also just remove it and stick with the profile setting.
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## Validation Steps Performed
1. Set "confirmCloseAllTabs" to false in my profile.json file.
2. Opened a 2nd tab.
3. Closed the window
4. Observed that there was no confirmation before the window closed.
5. Set "confirmCloseAllTabs" to true
6. Repeat steps 2 and 3
7. Observe that there was a confirmation before the window closed.
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